Software size is the basement for other measurements.Functional size method can be used to estimate the size of the software in the early lifecycle.However along with the realtime software system's wide application,the traditional function point size method is no more applicable.COSMICFFP is the second-generation method available to realtime system.In the paper the COSMICFFP method is introduced and how to use it to produce the size of the software modeled by UML for realtime is presented.